PAIN DURING AND AFTER SEX

During sex this evening with my boyfriend, I was perfectly fine and it felt AMAZING, he came once inside me and then kept going (which we have done before), but as we kept going it became more and more painful. Finally, we just decided to stop because I could hardly bare the pain anymore. 
After sex I went to the bathroom to go urinate and I notice my outter lips and my clit were super swollen and it hurt unless I was putting pressure on it. (I had no pain while urinating.) 
Also, we had sex one other time today and I was fine. 
Is this just dryness? STI? UTI? Blood flow? Semen allergy? (I looked it up, it's a thing)